1. Field of the Invention
The present invention relates to an image processing apparatus and a control method thereof, and more particularly to an image processing apparatus that dynamically controls a brightness of an image, and a control method thereof.
2. Description of the Related Art
Japanese Patent Application laid-open No. 2003-233344 discloses obtaining corrected image data by performing a correction on input image data, and controlling a display brightness by multiplying a gain by the input image data or the corrected image data. Japanese Patent Application laid-open No. 2003-233344 also discloses a brightness control circuit having a feedback configuration.
Further, Japanese Patent Application laid-open No. 2003-233344 and Japanese Patent Application laid-open No. 2004-246211 disclose a method of reducing a reaction speed of a gain variation during adaptive gain control in which a gain is calculated for each frame. In this method, the reaction speed of the gain variation is reduced by performing filter processing on the gain, and as a result, a reduction in image quality caused by gain variation is suppressed. Furthermore, Japanese Patent Application laid-open No. 2004-246211 discloses a method of increasing the reaction speed of the gain variation when a scene change is detected by modifying the filter processing performed on the gain.
FIG. 7 is a block diagram showing a brightness control circuit having a feedback configuration.
Input image data 303 are multiplied by a gain 305 to obtain brightness-controlled image data 306. The brightness-controlled image data 306 are subjected to correction processing by a correction block 302 and then output as output image data 301. The output image data 301 are input into a display panel (not shown) and a gain calculation block 304. The gain calculation block 304 determines the gain 305 on the basis of the output image data 301 for one frame. The gain 305 is then multiplied by the input image data 303 of the frame that follows the frame for which the gain 305 was determined.
An advantage of a feedback configuration is that brightness control is performed in front of the correction block 302 such that the image data corrected in the correction block 302 can be output as is to the display panel. By inputting this output into a modulation circuit, brightness control can be performed at the same time as the corrected image is displayed as is, and this is extremely favorable.
However, a disadvantage of a feedback configuration is that the image data used to determine the gain are different to the image data to be multiplied by the determined gain. This problem is known as a feedback control delay. Image data for a single frame are required to calculate the gain. Therefore, the gain is calculated after the image data for a single frame are input, or in other words at a frame changeover timing. On the other hand, gain multiplication is performed as soon as the image data are input. At this time, the gain multiplied by the input image data for an Nth frame is the gain calculated on the basis of the input image data for an N−1th frame.
When the output image is a still image or a moving image exhibiting little brightness variation, a rate of change in the determined gain is small, and therefore a feedback control delay is not greatly problematic. However, when the output image exhibits great brightness variation, for example during a still image changeover, a scene change, or the like, the rate of change in the gain increases. When the rate of change in the gain increases, effects caused by the feedback control delay may become more pronounced. For example, when a dark image is changed over to a light image, a screen appears to flash momentarily. This phenomenon will now be described using FIGS. 5A and 5B. FIG. 5A shows examples of input images and FIG. 5B shows examples of output images generated by conventional brightness control. Further, FIG. 5A shows gains (IGAIN) calculated from the respective images, while FIG. 5B shows gains (FBGAIN) that are multiplied by the respective images. FBGAIN is delayed by one frame relative to IGAIN. When a white image is input following input of consecutive black images, a gain calculated from a black image (frame 2) is multiplied by the white image (frame 3) immediately following the changeover. A value A of the gain calculated from the black image is larger than a value B of an optimum gain for the white image, and therefore the output image of frame 3 is lighter than a desired brightness. This increased brightness appears as a flash that disturbs a viewer.
Another problem arises in relation to an output capacity of a power supply. FIG. 5D shows temporal variation in a consumed current of an image display apparatus. When the output image generated by the conventional brightness control of FIG. 5B is displayed, the consumed current varies sharply at problematic frame 3. To ensure that the power supply continues to perform output with stability despite this sharp variation in the consumed current, the output capacity of the power supply must be increased. As a result, it becomes difficult to reduce the size, thickness, and cost of the image display apparatus, which is undesirable.
Note that the problems caused by the delay described above can be solved by providing a feedforward configuration in which the gain determined from the corrected image data is multiplied by the corrected image data themselves. With a feedforward configuration, however, the correction processing is implemented on the image data prior to brightness control, and the gain is multiplied by the post-correction image data. Hence, it is difficult to realize a favorable degree of correction precision. In particular, when the correction processing involves nonlinear conversion of an image data value, the correction precision deteriorates dramatically. Therefore, in terms of image quality, brightness control employing a feedback configuration is preferable to brightness control employing a feedforward configuration.